The use of gypsum in the production of building materials and products will always be justified by the property of this material to quickly gain strength without heat treatment. The main problem facing the wider use of gypsum is its lack of water resistance. When moistened, gypsum products significantly reduce their strength, therefore, despite all their positive qualities, gypsum materials are almost not used in structures subject to significant moisture.
Recently, domestic manufacturers and researchers have developed several ways to increase the water resistance of gypsum materials.
One of the methods is the introduction of Portland cement in semi-aquatic gypsum in an amount of 15-30% (or more) together with active hydraulic additives. The resulting mixed three-component (gypsum + Portland cement + hydraulic additive) binders are fast-setting and initial hardening of semi-aquatic gypsum, as well as the ability to hydraulic harden (like cements) in a humid and even aqueous environment. The ability to control the processes of interaction of gypsum and Portland cement with the help of hydraulic additives has been proven by research. Hydraulic additives reduce the concentration of calcium hydroxide in aqueous solutions, which favorably affects the formation of calcium hydrosulfoaluminate and the resistance of products to these binders over time. However, not all regions of the country have raw materials suitable for the preparation of HCPV (gypsum-cement-pozzolanic binders). Thus, the use of spent silica gel will solve not only the raw material problem, but also the environmental one - by recycling multi-ton industrial waste.
During the experiments, the influence of various factors on the properties of samples based on GCPV was studied and the amount of CaO in the mixture was determined. Earlier research conducted at IISS them. V.V. Kuybysheva, showed the possibility of increasing the water resistance of gypsum binders by mixing them with Portland cement and active hydraulic additives. The latter perform two main functions. The first of these is reduced to a decrease in the concentration of calcium hydroxide in an aqueous solution to such limits when, due to an increase in the solubility of alumina, ettringite begins to occur mainly in the aquatic environment, and not on the surface of cement particles, and then it contributes not to destruction but to hardening of the existing structure of cement stone . In this case, a positive role is played by all components of hydraulic additives capable of interacting with calcium oxide hydrate to form sparingly soluble substances. The second function of hydraulic additives is to bind calcium sulfates and aluminates to complex compounds that are less soluble compared to the starting materials.
Thus, HCPVs are characterized by a continuous increase in strength during prolonged exposure to humid conditions, while the strength of pure gypsum products decreases and decreases by 2.5–3.0 times by the age of one month.
Mixtures containing 50–70% gypsum, 20–25% cement and 15–30% hydraulic additives have adequate water resistance. Such mixed binders are characterized by significant strength (after 1–7 days) and the ability to hydraulically harden for long periods (up to 1–2 years or more).
The use of more active tripoli or other hydraulic additives also positively affects the properties of the binder. In particular, the water resistance of the binder, characterized by the ratio of compressive strength of water-saturated samples to the strength of dried (K3), increases from 0.60–0.65 to 0.80 and higher.
The amount of hydraulic additive should be assigned in such a way that the concentration of calcium oxide in the solution for 7 days. from the beginning of hardening it did not exceed 0.9 g / l, and in the first 3 days. - 1 g / l. With a lower concentration, the properties of HCPV improve. In this case, binders containing low aluminate cement will have the best performance.
The most characteristic additive is tripoli. However, studies have shown that its introduction into gypsum cement compositions is not a sufficiently effective technological method that provides optimal conditions for stone formation. Unlike tripoli, white soot (amorphous silica) has a greater reactivity. So, with a decrease in the content of semi-aquatic gypsum in the system, the plastic strength of the crystallization structure of the material does not decrease, as is the case with tripoli. On the contrary, the rapid growth of this strength was established, reaching maximum values when the content of gypsum gypsum in the amount of 60–70% of the mass of the dispersed phase. With the same content of semi-aquatic gypsum in the system with an increase in the amount of white soot up to 10%, the plastic strength of the material structure increases. The maximum growth rate is observed at the optimum content of semi-aquatic gypsum.
With respect to tripoli there is no such pattern of change in plastic strength. On the contrary, both with a decrease in the content of gypsum binder and with an increase in tripoli content, the plastic strength of the crystallization structure decreases.
The introduction of amorphous silica into gypsum-cement compositions is undoubtedly more effective than the use of active mineral additives like tripoli. To achieve the optimal structure of the stone with maximum strength, the consumption of white soot should be 10%, and for the necessary stability of this structure - 15% by weight of Portland cement. It can be assumed that the addition of silica gel, which is an amorphous silica, will have the same effect on gypsum-cement-pozzolanic systems as white soot.
Also, studies have shown that silica gel can be used as an additive - as a production waste, which is used to clean gas from oil products. The use of silica gel allows you to increase the water resistance of the material, regardless of whether pure or processed silica gel is used. Using this technology, it is possible to obtain a water resistance coefficient of a composition higher than 0.8, and then the material can be used not only for air but also for wet conditions, as well as when exposed to water.
When using Portland cement to increase the water resistance of gypsum, it is recommended that electrolytes capable of neutralizing calcium hydroxide be introduced into gypsum cement compositions. This is an effective technological tool that improves the conditions for the formation of artificial stone. Electrolytes prevent the inclusion of the structure of unhydrated Portland cement particles into the crystallization framework, which reduce its stability. Alkali metal carbonates, in addition, intensify the processes of hydration of Portland cement, due to which the hardening rate of gypsum cement compositions increases significantly. In quantity, they should correspond to the stoichiometric ratio to the free calcium oxide present in Portland cement.
It is also recommended a short-term steaming of gypsum-cement building products before drying, which provides a significant improvement in the quality of products: ceteris paribus, strength growth of more than 20% is achieved. It was found that the optimum temperature regime for the preparation and hardening of gypsum-cement compositions is 35–40 ° C. The positive effect of short-term steaming or the use of warm mixtures at this temperature is due to the improvement of the hydration hardening conditions of the cement component of the composition.
As a result of optimizing the conditions for the formation of gypsum cement, it seems possible to obtain materials that are comparable in physical and mechanical properties and durability to wall materials on Portland cement. The use of a complex binder from Portland cement and gypsum binder has, without a doubt, great advantages. Thanks to a quick set of strength, there is no need for heat treatment of products, which saves a large amount of heat going to this operation. There is considerable economic benefit when replacing part of the cement with gypsum, due to its relatively low cost. The environmental problem of the disposal of spent silica gel is also being addressed.